Various circuits using an amplifying effect of a vacuum tube are proposed in various fields. Traditionally, in order to fulfill the amplifying effect of the vacuum tube, a rated voltage is supplied to a heater to flow a large current therethrough to emit electrons from the cathode of the vacuum tube. An amount of current flowing toward the plate of the vacuum tube caused by the emitted electrons is controlled by a negative voltage supplied to the grid terminal. In addition, in order to extract a large amount of controllable plate current, unlike the conventional controlling method of the vacuum tube, a method of supplying a positive voltage to the grid terminal has been proposed. Such a technology is disclosed, for example, by Japanese Patent Laid-Open Publication No. 59-66210, at pages 1-2 and FIG. 1.
According to the proposed technology, a larger amount of controllable plate current can be extracted by supplying a positive voltage to the grid terminal than the conventional method where a negative voltage is supplied to the grid terminal. However, in the proposed technology, in order to achieve an electron emission action, a rated voltage has to be supplied to the heater and a high voltage has to be supplied to the plate terminal. Thus, the vacuum tube circuit in the proposed technology consumes a relatively large amount of power.